This invention relates to communication systems, and more particularly to methods of increasing capacity in dense wavelength division multiplexing (DWDM) systems.
Technological interest in dense wavelength division multiplexing systems is fast increasing. DWDM provides a new direction for solving capacity and flexibility problems in optical communications and networking. It offers a very large transmission capacity and new novel network architectures, as described, for example, in the following papers: C. A. Brackett, “Dense Wavelength Division Multiplexing Networks: Principles and Applications.” IEEE J Select. Areas Commun., vol. 8, pp. 948-964. 1990; and C. A. Brackett, A. S. Acampora, I. Sweitzer. G. Tangonan. M. T. Smith, W. Lennon. K. C. Wang, and R. H. Hobbs. “A Scalable Multiwavelength Multihop Optical Network: A Proposal for Research on All-Optical Networks.” J. Lightwave Technol., vol. II. pp. 736-753, May/June 1993. Major components in DWDM systems are the wavelength multiplexers and demultiplexers. Commercially available components are based on fiber-optic or micro-optic techniques. See, e.g., E. C. M. Pennings, M. K. Smit, and G. D. Khoe, ‘Micro-Optic versus Waveguide Devices—An Overview,’ invited paper, in Proc. Fifth Micm. Opdcs. Conf 1995, Hiroshima. Japan. Oct. 18-20. 1995, pp. 248-255; and E. C. M. Pennings, M. K. Smit. A. A. M. Staring, and G.-D. Khoe. “Integrated-Optics versus Micro-Optics—A Comparison.” Integrated Photonics Research IPR '96. Boston. MA. Tech. Dig. vol. 6. Apr. 29-May 2, 1996. pp. 460-463.
Research on integrated-optic (de)multiplexers has increasingly been focused on grating-based and phased-array (PHASAR) based devices (also called arrayed waveguide gratings. See, e.g., J. P. Laude, Wavelength Division Multiplexing, Prentice Hall, N.Y., 1993; and M. K. Smit, “New Focusing and Dispersive Planar Component based on an Optical Phased Array.” Electron. Lett., vol. 24, no. 7, pp. 385-386, March 1988. Both types of devices are imaging devices, i.e., they image the field of an input waveguide onto an array of output waveguides in a dispersive way. In grating-based devices a vertically etched reflection grating provides the focusing and dispersive properties required for demultiplexing. In phased-array based devices these properties are provided by an array of waveguides, the length of which has been chosen such as to obtain the required imaging and dispersive properties. As phased-array based devices are realized in conventional waveguide technology and do not require the vertical etching step needed in grating-based devices, they appear to be more robust and fabrication tolerant. The first devices operating at short wavelengths were reported by Vellekoop and Smit, e.g., in the above-referenced paper by Smit and in the following papers: B. Verbeek and M. K. Smit, “Phased Array Based WDM devices” in Proc. Eur. Conf. on Optical Communication (ECOC'95), Brussels. Belgium. Sep. 17-21, 1995, pp. 195-202; and A. R. Vellekoop and M. K. Smit, “Low-Loss Planar Optical Polarization Splitter with small dimensions,” Electron. Lett. ˜Vol. 25. pp. 946-947, 1989. Takahashi et al. reported the first devices operating in the long wavelength window. See H. Takahashi. S. Suzuki. K. Kaco. and I. Nishi, “Arrayed-Waveguide Grating for Wavelength Division Mult/Demultiplexer with Nanometer Resolution,” Electron. Lett., Vol. 26., no. 2, pp. 87-88, Jan. 1990. Dragone extended the phased-array concept from 1×N to N×N devices, as reported in C. Dragone. “An N×N Optical Multiplexer Using a Planar Arrangement of Two Star Couplers.” IEEE Photon. Technol. Lett. vol. 3. pp. 812-815, September 1991. This paper and all of the literature referenced above or elsewhere in this application are hereby incorporated by reference into the application.
Array Waveguide Grating (AWG) based multiplexers and demultiplexers are essentially the same. Depending on the direction of light wave propagation, the device can be used as either a multiplexer or a demultiplexer. For the sake of simplicity, the demultiplexer operation is illustrated here.
The AWG consists of two arrays of input/output waveguides, two focusing slab regions and the array grating waveguides. This is illustrated in FIG. 1. A single fiber containing the multi-wavelength input is connected to the array of input waveguides. The input multi-wavelength signal is evenly split among the input waveguides and the signal propagates through the input waveguide to reach the input focusing slab region. The light wave travels through the focusing slab and forms an interference pattern at the output end of the input-focusing slab. The light wave is then coupled into the array grating waveguides. Due to the path length difference of each array grating waveguide, a linear phase shift occurs in the light wave traveling through the array grating waveguide. The light wave is subsequently coupled into the output focusing slab. At the output end of the output focusing slab, the multi-wavelength input signal is split into different beams according to their wavelength.
To illustrate the wavelength demultiplexing mechanism, we first examine the path lengths and path difference between the array grating waveguides. The length of the array waveguides and the path length difference ΔL between two adjacent waveguides are chosen in such a way that the phase retardation for the light wave of the center wavelength passing through every array waveguide is 2 nm. So the phase front of the lightwave at the input end of the array waveguide is reproduced at the output end for the center wavelength, also assuming all the array waveguides are de-coupled. As the light beams propagate through the slab region, the constructive interference forms on the output side of the output-focusing slab. If only the center wavelength is applied, the constructive interference pattern turns out to be a single focused spot at the output side of the output-focusing slab. As the light waves of wavelengths other than center wavelength are passing through the array waveguides, their phase retardation is different from the center wavelength. In fact the phase retardation is linearly proportional to the difference in wavelength with respect to the center wavelength. That induces a linear phase difference in components of different wavelengths within the multiple wavelength input. Such linear phase difference results in a tilting effect for the wavefront as the light wave goes through the array grating waveguides and reaches the input side of the output-focusing slab. A unique phase front is created for each wavelength. As a result, each wavelength would be focused to different positions on the output side of the output focusing slab region. Then, each wavelength is coupled through the output waveguide. This completes the wavelength demultiplexing operation.
The following is an approximate analysis of the focusing and dispersion properties of the demultiplexing operation. In the input focusing slab region, the spacing between the end of adjacent waveguides on the input side is D1, the separation on the output side is d1, distance measured from the center of the input side is x1. The radius of the output curvature is f1. In the output focusing slab region, the spacing between the ends of adjacent waveguides connected to the array waveguide is d. The spacing between the ends of adjacent waveguides connected to the output waveguide is D. The radius of the output curvature is f. As mentioned earlier, the path differences between two adjacent waveguides is ΔL, and the phase retardation is 2mπ with respect to the center wavelength.
Since the processes in the two focusing slab regions are mirror images of each other, it is sufficient to consider the output focusing slab region. We focus upon the light beams passing through the ith and (i−1)th array waveguide. In order for the two light beams to interfere constructively, their phase difference should be multiples of 2πas they reach the output side of the focusing slab region. The condition for constructive interference is given by                                                                         β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                          (                                                f                  1                                -                                                                            d                      1                                        ⁢                                          x                      1                                                                            2                    ⁢                                          f                      1                                                                                  )                                +                                                    β                c                            ⁡                              (                                  λ                  0                                )                                      ⁡                          [                                                L                  C                                +                                                      (                                          i                      -                      1                                        )                                    ⁢                  Δ                  ⁢                                                                           ⁢                  L                                            ]                                +                                                    β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                          (                              f                +                                  dx                                      2                    ⁢                    f                                                              )                                      =                                                            β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                          (                                                f                  1                                +                                                                            d                      1                                        ⁢                                          x                      1                                                                            2                    ⁢                                          f                      1                                                                                  )                                +                                                    β                c                            ⁡                              (                                  λ                  0                                )                                      ⁡                          [                                                L                  c                                +                                  i                  ⁢                                                                           ⁢                  Δ                  ⁢                                                                           ⁢                  L                                            ]                                +                                                    β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                          (                              f                -                                  dx                                      2                    ⁢                    f                                                              )                                -                      2            ⁢            π            ⁢                                                   ⁢            m                                              (        1        )            where βs and βc denote the propagation constants in slab region and array waveguide, m is an integer, λ0 is the center wavelength of the multiple wavelength input, and Lc is the minimum array waveguide length. Subtracting common terms from Equation (1), we obtain                                                                         β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                                                            d                  1                                ⁢                                  x                  1                                                            f                1                                              +                                                    β                c                            ⁡                              (                                  λ                  0                                )                                      ⁢            Δ            ⁢                                                   ⁢            L                    -                                                    β                s                            ⁡                              (                                  λ                  0                                )                                      ⁢                          dx              f                                      =                  2          ⁢          π          ⁢                                           ⁢          m                                    (        2        )            When the conditionβS(λ0)ΔL=2πm  (3) or                               λ          0                =                                            n              c                        ⁢            Δ            ⁢                                                   ⁢            L                    m                                    (        4        )            is satisfied for λ0, the light input position x, and the output position x1 should satisfy the condition                                                         d              1                        ⁢                          x              1                                            f            1                          =                  dx          f                                    (        5        )            The spatial separation of the mth and (m+1)th focused beams for the same wavelength is called the free spectral range (FSR). It is obtained from Eq.(2) as                               X          FSR                =                                            λ              0                        ⁢            f                                              n              s                        ⁢            d                                              (        6        )            Number of available wavelength channels Nch is given by                               N          ch                =                                            X              FSR                        D                    =                                                    λ                0                            ⁢              f                                                      n                s                            ⁢              dD                                                          (        7        )            The resolution of the system is also obtained from Eq.(2) as                                           Δ            ⁢                                                   ⁢            x                                Δ            ⁢                                                   ⁢            λ                          =                                            N              c                        ⁢            f            ⁢                                                   ⁢            Δ            ⁢                                                   ⁢            L                                              n              s                        ⁢            d            ⁢                                                   ⁢                          λ              0                                                          (        8        )            Where Nc is the group index of the effective index nc, i.e. Nc=nc−λdnc/dλ.Setting Δx equal to D, we can get                               Δ          ⁢                                           ⁢          L                =                                            n              s                        ⁢            dD            ⁢                                                   ⁢                          λ              0                                                          N              c                        ⁢            f            ⁢                                                   ⁢            Δ            ⁢                                                   ⁢            λ                                              (        9        )            
Two examples of the results obtained with phasar simulation at a center wavelength of 1.550 micrometers, and channel spacing of 0.8 micrometers are given in FIGS. 2 and 3 to illustrate how the number of channels are limited. In FIG. 2, there are only 16 channels, and the second order channels on either side of the central channels do not overlap with the central channels, and hence the design works fine. On the other hand, in FIG. 3, there are 64 channels, and the second order channels on either side of the central channels start overlapping with the central channels, and hence the design starts becoming critical. In this particular case, the number of channels could not be increased any further. Currently, phasar devices being marketed have of the order of 40 channels.